There are two problems in your code:
1) The method of "https://api-sandbox.inmobi.com/v1.0/generatesession/generate" is not POST, it's a GET method
2) CURLOPT_HTTPHEADER is expecting an array, not a key-value pair.
Sample Code:
$ch = curl_init();
curl_setopt($ch, CURLOPT_URL, "https://api-sandbox.inmobi.com/v1.0/generatesession/generate");
curl_setopt($ch, CURLOPT_RETURNTRANSFER, 1);
//curl_setopt($ch, CURLOPT_POST, true);
curl_setopt($ch, CURLOPT_HEADER, truea);
$data = array(
'username: XXXXXXXX',
'password: XXXXXXXX',
'secretKey: XXXXXXXXXXXXXXX'
);
//curl_setopt($ch, CURLOPT_POSTFIELDS, $data);
curl_setopt($ch, CURLOPT_HTTPHEADER, $data);
$output = curl_exec($ch);
$info = curl_getinfo($ch);
curl_close($ch);
echo "<pre>";
print_r( $info);
print_r($output);